1. Field of the Invention
The present invention relates to analog integrated circuits. In particular, the present invention relates to DC offset correction for direct baseband conversion.
2. Description of Related Art
The direct conversion of a radio frequency (RF) signal to a baseband signal in a receiver system typically uses a mixer to multiply the RF signal by a signal from a local oscillator (LO) operating at a frequency close to the RF frequency.
A mixer is basically a multiplier which generates a signal with frequency components at the difference and sum frequencies. Mixing is achieved by applying the RF signal and the LO signal to a non-linear element, which can be a diode or a transistor. There are many types of mixers: single-ended, balanced or anti-parallel, double balanced, and double-double balanced. The selection of a proper mixer requires a trade-off between isolation performance and low conversion loss.
When the receiver is implemented in a single tuner integrated circuit (IC), the LO signal tends to leak into the RF input port of the tuner IC. This leakage causes self mixing of the local oscillator, which is translated into a large DC offset at the mixer output. This tends to drive the baseband amplifier out of its dynamic range, causing distortion to the desired baseband signals.
The DC offset may be filtered out by a high pass filter or a bandpass filter at the mixer output. However, such a filter usually has to have a sharp roll-off frequency response to maintain the quality of the baseband output signal. The design of such a sharp roll-off filter is complex and expensive, and further would require large value components which would not be suitable for monolithic implementation, and thus is not suitable for implementation in a single tuner IC.
Therefore, it is desirable to have a method and apparatus to provide a DC offset correction in direct baseband converter.
The present invention discloses a method and apparatus for correcting the DC offset caused by self-conversion of the oscillator signal in an RF-to-baseband converter. The circuit comprises: a subtractor for subtracting an amplified DC offset signal from a filtered mixer signal to generate a corrected signal; a baseband amplifier coupled to the subtractor to receive the corrected signal to generate a baseband output signal; a low-pass filter coupled to the baseband amplifier to generate a DC offset signal representative of the DC offset; and a DC offset amplifier coupled to the low-pass filter and the subtractor to generate the amplified DC offset signal.